In a power conversion system such as a variable speed, constant frequency (VSCF) power generating system, a brushless, three-phase synchronous generator operates in a generating mode to convert variable speed motive power supplied by a prime mover into variable frequency AC power. The variable frequency power is rectified and provided over a DC link to a controllable static inverter. The inverter is operated to produce constant frequency AC power, which is then supplied over a load bus to one or more loads.
As is known, a generator can also be operated as a motor in a starting mode to convert electrical power supplied by an external AC power source into motive power which may in turn be provided to the prime mover to bring it up to self-sustaining speed. In the case of a brushless, synchronous generator including a permanent magnet generator (PMG), an exciter portion and a main generator portion mounted on a common shaft, it has been known to provide power at a controlled voltage and frequency to the armature windings of the main generator portion and to provide field current to the main generator portion via the exciter portion so that the motive power may be developed. This has been accomplished in the past, for example, using two separate inverters, one to provide power to the main generator portion armature windings and the other to provide power to the exciter portion. Thereafter, operation in the generating mode may commence whereupon DC power is provided to the exciter field winding.
Cook, U.S. Pat. No. 4,786,852, assigned to the assignee of the instant invention, discloses a power conversion system including a starting arrangement in which a brushless generator is operated as a motor to bring an engine up to self-sustaining speed. A rectifier bridge of a VSCF system is modified by adding transistors in parallel with the rectifiers of the bridge and the transistors are operated during a starting mode of operation to convert DC power provided on a DC link by a separate VSCF system or auxiliary power unit into AC power. The AC power is applied to armature windings of the brushless generator to cause a rotor of the generator to be accelerated.
Shilling, et al., U.S. Pat. No. 4,743,777 discloses a starter/generator system including a brushless, synchronous generator. The system is operable in a starting mode to produce motive power from electrical power provided by an external AC power source. An exciter of the generator includes separate DC and three-phase AC field windings disposed in a stator. When operating in a starting mode at the beginning of a starting sequence, the AC power developed by the external AC power source is directly applied to the three-phase AC exciter field windings. The AC power developed by the external AC source is further provided to a variable voltage, variable frequency power converter which in turn provides a controlled voltage and frequency to the armature windings of a main generator. The AC power provided to the AC exciter field windings is transferred by transformer action to exciter armature windings disposed on a rotor of the generator. This AC power is rectified by a rotating rectifier and provided to a main field winding of the generator. The interaction of the magnetic fields developed by the main generator field winding and armature windings in turn causes the rotor of the generator to rotate and thereby develop the desired motive power. When the generator is operated in a generating mode, switches are operated to disconnect the AC exciter field windings from the external AC source and to provide DC power to the DC exciter field winding. The power converter is thereafter operated to produce AC output power at a fixed frequency.
Compoly, et al., U.S. Pat. No. 3,775,662, discloses a power converter which includes four inverter bridges that are in turn coupled to wye-and delta-connected primary windings of a transformer. Secondary windings of the transformers are connected in series to sum three-phase outputs developed in the windings. The bridges are controlled such that 24-four step output phase voltages are produced.